As distinct from a primary cell such as a dry battery, and a secondary battery such as a lead storage battery, a fuel cell is capable of continuously producing electric power by being continuously supplied with a fuel such as hydrogen and an oxidant such as oxygen. Such a fuel cell is high in electric power generation efficiency, is not largely affected by the size of the system, and causes less noises and vibrations. For these reasons, the fuel cell is expected as an energy source covering various applications and scales. Thus, fuel cells have been under development specifically as polymer electrolyte fuel cells (PEFC's), alkaline fuel cells (AFC's), phosphoric acid fuel cells (PAFC's), molten carbonate fuel cells (MCFC's), solid oxide fuel cells (SOFC's), biofuel cells, and the like.
A description will be given by taking a polymer electrolyte fuel cell as one example of fuel cells. Such a polymer electrolyte fuel cell includes a plurality of single cells stacked one on another each with an electrode referred to as a separator (or a bipolar plate) therebetween. Each single cell includes a solid polymer electrolyte membrane, and an anode electrode and a cathode electrode interposing the membrane therebetween.
The material for the separator for a fuel cell is required to have a characteristic of low contact resistance (meaning that a voltage drop occurs due to the interface phenomenon between the electrode and the separator surface), to be kept for a long period during use as a separator. From the viewpoints of this point and the processability and the strength in combination, a study has been conventionally made on applications of a metal material such as an aluminum alloy, stainless steel, a nickel alloy, or a titanium alloy.
For example, JP-A No. 10-228914 has a description on a separator for a fuel cell to the effect that stainless steel is used as a base material, and a gold plating is applied to the surface for manufacturing thereof. Whereas, for example, JP-A No. 2001-6713 describes as follows: stainless steel or a titanium material is used as a base material, and a noble metal or a noble metal alloy is deposited on the surface thereof; alternatively, after removing an oxide film on the base material surface, a noble metal or a noble metal alloy is deposited thereon. Whereas, for example, JP-A No. 2006-97088 describes as follows: a titanium material is used as a base material, and an oxide film on the surface is removed, and then, 1- to 100-nm island-like gold plating portions are interspersed thereon.
Each separator described in JP-A No. 10-228914, JP-A No. 2001-6713, and JP-A No. 2006-97088 uses a corrosion resistant metal such as stainless steel or titanium as a base material. For this reason, in a polymer electrolyte fuel cell using a hydrogen gas as a fuel, the separator is less susceptible to corrosion, and can keep a low contact resistance to a certain degree. However, in a direct methanol type fuel cell using a methanol aqueous solution as a fuel, which has been under development from the viewpoint that the fuel cell can be reduced in size, when methanol reacts with water on an electrode catalyst to form hydrogen, highly corrosive formic acid is also formed. Stainless steel or titanium is excellent in corrosion resistance because is forms a passive film on the surface. However, formic acid is a reducing acid, and hence it reduces and dissolves the passive film, thereby to corrode stainless steel or titanium. Therefore, when a gold-plated layer or the like includes pinholes, corrosion of the base material proceeds therefrom. Particularly, when the proportion of pinholes is large, corrosion proceeds from the pinhole areas at the interface between the base material immediately under the gold-plated layer or the like and the gold-plated layer or the like. As a result, much peeling of the gold-plated layer or the like occurs, resulting in an increase in contact resistance. This may lead to reduction of the performances of the fuel cell. Further, peeling of the gold-plated layer or the like causes corrosion to further proceed. As a result, the solid polymer electrolyte membrane may be deteriorated by metal ions dissolved from the base material. In contrast, it can be considered that the gold-plated layer or the like is increased in thickness to minimize pinholes. However, noble metals such as gold are expensive. Further, the plating time increases, resulting in reduction of productivity. This unfavorably incurs a higher cost of the separator.
For the separators described in JP-A No. 2001-6713 and JP-A No. 2006-97088, in order to enhance the electric conductivity, the oxide film on the surface of the base material made of titanium is removed. Then, in order to prevent the formation of an oxide film again, an electric conductive layer of a noble metal, an electrically conductive resin, or the like is provided under a prescribed conditions such as a vacuum atmosphere or a reducing atmosphere. This is for the following reason. Removal of the oxide film can enhance the electric conductivity. On the other hand, hydrogen becomes more likely to penetrate into the base material, which causes a concern about embrittlement of the base material due to penetration of hydrogen in the long view.
Herein, the separator for a fuel cell is required to have both high electric conductivity and high corrosion resistance. However, the separator for use on the methanol electrode side is further required to have a hydrogen absorption resistance: the separator is further required to be resistant to mechanical embrittlement due to absorption of hydrogen formed due to decomposition of methanol. However, a pure titanium material or a titanium alloy material tends to absorb hydrogen to be embrittled as the characteristic of the material. A common method for inhibiting the hydrogen absorption includes a method in which a titanium oxide layer is formed on the surface. However, the titanium oxide layer is an insulation layer. Therefore, with this method, the electric conductivity is reduced.